classical gene therapy

The classical gene therapy approach is to introduce a
functioning gene into cells in order to produce a protein
product that is missing or defective, or to supply a gene that
has a novel function. This type of gene augmentation approach
could be appropriate for conditions that are due to deficiency
of a particular gene product where the disease process may be
reversed without very high levels of gene expression being
required. Autosomal recessive and X linked recessive disorders
are likely to be the best candidates for this approach since most
are due to loss of function mutations leading to deficient or
defective gene products. Augmentation gene therapy is not
likely to be successful in autosomal dominant disorders, since
affected heterozygotes already produce 50% levels of normal
gene product from their normal allele. In these cases, gene
therapy is not likely to restore gene product production to
levels that will have a therapeutic effect. In neoplastic disorders
the classical gene therapy approach aims to introduce genes
whose products help to kill malignant cells. The genes
introduced may produce products that are toxic, act as
prodrugs to aid killing of cells by conventionally administered
cytotoxic agents, or provoke immune responses against the
neoplastic cells.